Stents are tubular structures that are implanted inside bodily conduits, blood vessels or other body lumens to widen and/or to help keep such lumens open. Typically, stents are delivered into the body while in a compressed configuration, and are thereafter expanded to a final diameter once positioned at a target location within the lumen. Stents are often used following or substituting for balloon angioplasty to repair stenosis and to prevent future restenosis and, more generally, may be used in repairing any of a number of tubular body conduits such as those in the vascular, biliary, genitourinary, gastrointestinal, respiratory and other systems. Exemplary patents in the field of stents formed of wire, for example, include U.S. Pat. Nos. 5,019,090 to Pichuk; U.S. Pat. No. 5,161,547 to Tower; U.S. Pat. No. 4,950,227 to Savin et al.; U.S. Pat. No. 5,314,472 to Fontaine; U.S. Pat. Nos. 4,886,062 and 4,969,458 to Wiktor; and U.S. Pat. No. 4,856,516 to Hillstead; each of which is incorporated herein by reference. Stents formed of cut stock metal, for example, are described in U.S. Pat. No. 4,733,665 to Palmaz; U.S. Pat. No. 4,762,128 to Rosenbluth; U.S. Pat. No. 5,102,417 to Palmaz and Schatz; U.S. Pat. No. 5,195,984 to Schatz; WO 91 FR013820 to Meadox; and WO 96 03092 to Medinol, each of which is incorporated herein by reference. Bifurcating stents are described in U.S. Pat. No. 4,994,071 to MacGregor, and commonly-assigned U.S. patent application Ser. No. 08/642,297, filed May 3, 1996, each of which is incorporated herein by reference.
For stents to be effective, it is essential that they be accurately positioned at a target location within a desired body lumen. This is especially true where, for example, multiple stenting is required with overlapping stents to cover excessively long regions or bifurcating vessels. In these and other cases, it is often necessary to visually observe the stent both during placement in the body and after expansion of the stent. Various approaches have been attempted to achieve such visualization. For example, stents have been made from radiopaque (i.e., not allowing the passage of x-rays, gamma rays, or other forms of radiant energy) metals, such as tantalum and platinum, to facilitate fluoroscopic techniques. One of the potential problems with such stents, however, is that a useful balance of radiopacity and stent strength is difficult, if not impossible, to achieve. For example, in order to form such a stent of adequate strength, it is often necessary to increase stent dimensions such that the stent becomes overly radiopaque. Consequently, fluoroscopy of such a stent after deployment can hide the angiographic details of the vessel in which it is implanted, thus making it difficult to assess problems such as tissue prolapse and hyperplasia.
Another technique that has been used to achieve the visualization of stents is the joining of radiopaque markers to stents at predetermined locations. The joining of the stent and marker materials (e.g., stainless steel and gold, respectively), however, can create a junction potential or turbulence in blood and thus promote thrombotic events, such as clotting. Consequently, the size of the markers is minimized to avoid this problem, with the adverse effect of greatly decreasing fluoroscopic visibility and rendering such visibility orientation-sensitive.
Yet another technique that has been used to achieve the visualization of stents is to simply increase the thickness of such stents to thereby increase radiopacity. Overly thick stent struts, however, effectively create an obstruction to blood flow. In addition, design limitations for stents having thick struts often result in large gaps between these struts, thus decreasing the support of a surrounding lumen. Furthermore, overly thick stent struts could adversely affect stent flexibility.
There is thus a need for the increased radiopacity of stents without sacrificing stent mechanical properties or performance. The coating of stents with radiopaque materials is described in U.S. Pat. No. 5,607,442 to Fishell et al. According to this patent, the disclosed radiopaque coating is much thicker on longitudinal stent members when compared with radial stent members such that only the longitudinal stent members are visible during fluoroscopy.